The disclosed invention generally relates to angle of arrival determining systems which sense irradiation by a remote source of electromagnetic energy where the radiated energy beam is substantially coherent and or parallel, and more particularly is directed to an angle of arrival determining system which provides reliable angle of arrival determination in the presence of atmospheric scintillation.
Angle of arrival determining systems are used, for example, in military vehicles such as aircraft or tanks to detect incident radiation beams emanating from a remote source which may be a threat to the vehicle. In such cases, the purpose of the system is to determine the direction from the vehicle to the threat and so warn the vehicle operators. Such systems may be used in conjunction with other electromagnetic sensor systems which are designed to determine the type of electromagnetic source from which the incident radiation emanates. Sensors for laser target designators used in vehicles or from fixed locations also may benefit from such systems.
One general type of angle of arrival determining system includes a sensor assembly having an array of many detectors arranged in a semicircle behind an aperture in such a way that, depending on the angle of arrival of the incident energy beam, different detectors in the array are illuminated. Detector outputs are evaluated and the angle of arrival is determined by association with the position of the detector having the greatest output.
This system is very complex electronically and expensive as a result.
Another known system includes a sensor having two detectors separated by two reflecting surfaces and positioned in such a way that the relative amount of energy falling on each detector varies with the angle of arrival of the energy beam. In such a device, the ratio of output of one detector to the other is theoretically proportional to the angle of arrival. Generally, two sensor assemblies or two sets of detector arrays are required for determination of the compound angle of arrival. They may be arranged in orthogonal planes, each measuring one component of the total angle.
Such a system is generally less expensive to manufacture than the multiple-detector system described above. However, presence of atmospheric scintillation due to changing refraction of the atmosphere (the mechanism which, for example, makes stars twinkle, or creates mirages) adversely affects the reliable determination of the angle of arrival. When the energy beam passes through an atmospheric path typical in many practical applications of such systems, scintillation creates rapidly varying, non-uniform, concentrated energy peaks and valleys in segments of the beam. This condition occurs frequently. In such systems atmospheric scintillation is known to make the beam erroneously appear to come from another direction. This occurs because the beam energy in such systems is spread over two or more detectors and scintillation causes non-uniformity of the energy distribution within the beam pattern. As a result, detector outputs are not associated with or indicative of the angle of arrival.
The ratioing system described above does not solve the problem of providing reliable and presence of atmospheric scintillation. Such a solution would satisfy a long felt need for aircraft, military and other vehicle applications of laser detection systems. The disclosed ratioing system does provide such a solution.